Comparing amorphous silica, short-range-ordered silicates and silicic acid species by FTIR

There is increased interest in the terrestrial silicon cycle in the last decades as its different compounds and species have large implications for ecosystem performance in terms of soil nutrient and water availability, ecosystem productivity as well as ecological aspects such as plant–microbe and plant-animal feedbacks. The currently existing analytical methods are limited. Fourier-transform infrared spectroscopy (FTIR) analysis is suggested being a promising tool to differentiate between the different Si species. We report here on the differentiation of varying Si-species/Si-binding (in synthetic material) using FTIR-analyses. Therefore, we collected FTIR-spectra of five different amorphous silica, Ca-silicate, sodium silicate (all particulate), a water-soluble fraction of amorphous silica and soil affected by volcanic activity and compared their spectra with existing data. A decrease of the internal order of the materials analyzed was indicated by peak broadening of the Si–O–Si absorption band. Peak shifts at this absorption band were induced by larger ions incorporated in the Si–O–Si network. Additionally, short-range ordered aluminosilicates (SROAS) have specific IR absorption bands such as the Si–O–Al band. Hence, SROAS and Si phases containing other ions can be distinguished from pure amorphous Si species using FTIR-analyses.

www.nature.com/scientificreports/ different Si species such as different amorphous silica, quartz sand, silica gel, sodium silicate, calcium silicate, SROAS or amorphous Al:Si-phases, and water soluble Si forms. Consequently, the aim of our study was to identify absorption bands in FTIR spectra that are characteristic for the (i) amorphous silica (Sipernat, ZEOfree, Aerosil) as compared to quartz sand, silica gel, sodium silicate (air dried standard solution) and SROAS and (ii) water soluble fractions of amorphous silica compared to those of particulate silica. Our hypothesis were that (i) the Si-O-Si band in FTIR of amorphous silica fraction shows up at a region that is different to the ones of respective SROAS or sodium silicate and (ii) the FTIR of water soluble fraction of amorphous silica shows up bands at different WN as compared to those of the respective particulate silica. For the current study, we define the different analyzed compounds as follow: amorphous silica (amSiO 2 ), the quadruple negatively charged and tetrahedral coordinated SiO 4 4− anion of the monomeric silicic acid H 4 SiO 4 (SiO 4 4orthosilicate) and the condensates of H 4 SiO 4 as polymeric silicic acid, silanol the -Si-OH group within a silicon dioxide component. Silicates are salts of silicic acid and occur naturally as silicate minerals.
For each of the above named species about 0.5 mg sample was mixed with 100 mg of KBr (spectroscopy grade; Merck), stored overnight in an desiccator over silica-gel (to prevent water uptake), then finely-ground in an agate mortar and pressed into pellets 24 that were finally analyzed by using a FTS135 (BioRad Corp, Hercules, CA, USA) (BioRad). All samples were analyzed in four replicates (n = 4). To obtain the FTIR spectrum of sodium silicate (Na 4 SiO 4 ) about 1 mL of a sodium silicate solution (Merck Chemicals GmbH, Darmstadt, Germany) was placed at a zinc-selinid plate, and dried to obtain a white solid, and was analyzed using the FTS135 (n = 4).
Additionally, 10 g of Sipernate 50 s (Evonik Resource Efficiency GmbH, Wesseling, Germany) was mixed with 1 L of water, treated for 15 min within an ultrasonic bath (35 kHz; RK106; Bandelin Electronics, Germany), to ensure dispersion as far as possible. Thereafter, 100 mL aliquots of the mixture were transferred into 6 polyethylene cylinders (16 mm in diameter and 160 mm in height) (n = 6), stored for 48 h to allow precipitation of the Sipernat 50 s particles from the supernatant, and 10 ml aliquots were taken from the upper 50 mm of the supernatant. These aliquots were shock frozen by using liquid nitrogen. The shock-frozen samples were freeze dried and analyzed as described above by using FTIR.
Each FTIR spectrum was recorded by 16 co-added scans at a resolution of 1 cm −1 in the region of 400-4000 cm −1 . All FTIR spectra were corrected against the atmosphere as background 25 , and smoothed using a "boxcar "-function (f = 105 for KBr-transmission technique and f = 25 for micro-FTIR; Win-IREZ software, BioRad). The spectra were baseline-corrected and normalized for the band at WN 1100 cm −124 .
The spectra were interpreted as follows: The bands at spectral regions between WN 3500-3300 cm −1 are characteristic for hydroxyl groups (O-H; stretching vibration) that are part of water molecule and silanol groups (Si-OH), the latter mostly located at the surface of the silica particles. The adsorption band at about 1630 cm -1 is attributed to the bending 19 or deformation mode of molecular coordinated water adsorbed to the Si-O-Si structure (H 2 O band; Kaya et al. 21 , Roulia et al. 26 ). While the bands at 1050-1100 cm −1 are the most significant spectral region (with regard to Si) and accompanying shoulders are attributed to asymmetric stretching vibrations of Si-O-Si. Lower internal order of the material under study may leads to peak broadening of the entire envelope 20 , denoted here as Si-O-Si band. Infrared spectra of SROAS showed an absorption band between 1018 and 975 cm −1 which shifted to lower WN with increasing Al:Si ratio and is attributed to stretching vibrations of the Si-O-Al bond (oxygen bridges formed by condensation of Si and Al hydroxides 19,27 . The bands at wavenumber (WN) 980 cm −1 are characteristic for Si-O stretching vibrations of Si-O-H (Si-OH band) groups, which are attributed to asymmetric vibration of Si-OH. An increase of this bands is linked to an increase of OH-groups which are located at the surface of polymerized silica while those at WN 798 cm −1 are attributed to symmetric stretching vibrations of Si-O-Si 28 and that at 467 cm −1 for Si-O-Si out of plane deformations. The intensity of the bands were each measured as a vertical distance from the maximum of the respective band to the baseline (as it was described for C-O-C bands in Kaiser et al. 29 ). Though, Al-OH bending vibrations occur at WN 590-570 cm −1 . A relative decrease of their intensities to the intensity of the Si-O-(Al) stretching region (WN 1018 and 975 cm −1 ) is caused by a decreasing Si content of the SROAS. Furthermore, Si-rich SROAS show an absorption maximum at 690 cm −1 , while the bands at WN 430-440 cm −1 are related to vibrations of Si-OH groups 19 . Thus, these spectral regions are suitable to distinguish SROAS from "pure" Si phases. Though, the bands within the fingerprint region at WN < 1000 cm −1 in the FTIR spectra of the studied silica samples can be used to identify specific Si-O components.

Results and discussion
The FTIR spectra of all Sipernat 50 s samples (Sip) (Fig. 1) showed absorption bands characteristic for stretching vibrations of O-H groups (blue bar; OH band) at WN 3500-3300 cm −1 , and Si-O-Si groups (yellow bar; Si-O-Si band) at WN of 1000-1100 cm -1 . Additionally, the spectra show a band at WN 1600-1650 cm −1 (blue arrow) which indicate the presence of molecular coordinated water within the SiO 2 -structure 30 . The 1650 cm −1 band is less intense for Sip50s as compared to the other samples. A broadening of this envelope, an increase in intensity and/or peak shifts to higher WN may be attributed to a more complex structure of the material under study, which might result in a higher binding affinity towards water 19 . However, Sip50S showed a smaller particle size (18 µm) as compared to Sip 50 or Sip 320. www.nature.com/scientificreports/ The FTIR spectra of Aerosil showed the lowest intensity of the O-H band, which is in accordance with the fact that Aerosil is a pyrogenic silica that may contain only small amount of water and OH-groups due to the conditions of synthesis. Pyrogenic or fumed silica is formed when silicon tetrachloride (SiCl 4 ) reacts in a hydrogen flame (2100 K) to form amorphous silicon dioxide (SiO 2 ) 31 . With this single spherical droplets of silicon dioxide form and is followed by particle growth through collision and coalescence forming larger droplets whereas the aggregation is occurring via cooling 31 .
As expected, the FTIR spectra of the Sipernat-and Aerosil-samples correspond mostly to that of silica gel (SG, Fig. 1: grey line). However, for the silica gel the Si-O-Si band is narrower as compared to that of Sipernat and Aerosil (Fig. 1, grey arrow). The spectra of Sipernat has of course more OH groups compared to the Aerosil due the formation process. The FTIR of quartz sand (QS, black line in Fig. 1) is also mostly comparable to that of the silica gel sample but shows a second maximum at the left-hand side of the Si-O-Si band (Fig. 1, grey arrow).
The FTIR spectra of ZEOfree (Fig. 2), a precipitate calcium-silicate, showed an additional band at WN of about 1500 cm −1 ( Fig. 2; red solid arrow) compared to the ones of Sipernat 50 and 320 ( Fig. 1) and that of Sip-ernat50s (green line in Fig. 2), a shift of the Si-O-Si band towards smaller WN, and a second maximum at the right hand side of the Si-O-Si band ( Fig. 2; black solid arrow). Molecular IR vibrations are a function of binding strength and the mass of the atoms involved. A band shift towards lower WN occurs as the mass of the atoms (here m > 28 u) increases. Furthermore, peak broadening may indicate a lower internal order of the material under study 18,22 . The additional band at about 1500 cm −1 in the FTIR of ZEOfree ( Fig. 2; red solid arrow) is in a similar range of WN as the SiO 4 4− band of sodium silicate (Fig. 2, grey line, Fig. 2: empty red arrow). Compared to ZEOfree the FTIR spectra of the Andosol soils show no absorption band at about 1500 cm −1 (Fig. 3a, red arrow). The Si-O-Si bands of the Andosol soils look similar to those of the SROAS studied by Parfitt 28 (Fig. 3b). However, the FTIR spectra of SROAS show a the shift of the Si-O-Si band towards lower WN ( Fig. 3b; Parfitt 32 ), which is found by Wada et al. 33 to increase with Al content. However, there is currently some inconsistency in the definition of allophane. The definition of allophane commonly used in soils science from Parfitt 32 stating that: ' Allophane is the name of a group of clay-size minerals with short-range order which contain silica, alumina and water in chemical combination' is misleading as allophanes are defined as particles consisting of spherical, hollow units 34,35 . Short-range ordered silica, amorphous aluminosilicates and also amorphous silica are phases not belonging to the group of allophanes 4 . Only Imogolith (Fig. 3b, line A) showed a second maximum of the Si-O-Si band like ZEOfree a. The FTIR spectra of SROAS studied by Wada et al. 33 (designated therein as allophanes) showed also absorption bands at WN 550-720 cm −1 (Fig. 3b) which are indicative for Si-O-Al bands. Most recently Lenhardt et al. 20 reported on IR properties of SROAS with varying Al:Si ratios and identified SROAS specific absorption bands at 590-570 cm −1 (Al-OH bending vibration) which decreased with increasing Si content and an absorption maximum at 690 cm −1 for Si-rich SROAS. In contrast to the SROAS (Fig. 3b), the silica samples studied here (Fig. 1) did not show absorption bands at this WN.
The FTIR spectra of the water-soluble fraction of Sipernat 50 s showed a smaller Si-O-Si band (ca. 1100 cm −1 ) and an additional band (Fig. 4c, bluearrow) at about 1400 cm −1 which is characteristic for Si-O − groups compared www.nature.com/scientificreports/ to the bulk Sipernat 50S sample (Fig. 4b). The additional band in the spectra of the water soluble Sipernat50s fraction is located at a lower WN (WN 1400 cm −1 ) as compared to that of ZEOfree (1500 cm −1 ; Fig. 4a) and Na 4 SiO 4 (1475 cm −1 ; Fig. 4d). The narrower Si-O-Si band in the spectrum of silica gel compared to Sipernat or Aerosil samples (Fig. 1, grey arrow) can be explained by differences in the specific surface area (SSA) of Sipernat and Aerosol samples (150-500m 2 g −1 ) compared to the silica gel and quartz sand samples. With increasing specific surface area, the number of Si-OH increases and the number of Si-O-Si groups located near to the surface is increasing. Such the  www.nature.com/scientificreports/ ratio between the number of Si-O-Si group located near to the surface (Fig. 5b) and that of the ones located in the particles center (Fig. 5a) will increase. This may be important since the presence of Si-OH groups is affecting the binding strength of neighbored Si-O-Si groups. This effect may decrease with increasing distance such that a closely neighbored Si-O-Si group will be affected more strongly (green colored bonds, Fig. 5c) than Si-O-Si groups that are located at larger distances (brownish to black colored bonds, Fig. 5c from the particles surface (i.e., within the center of a non-porous silica particle like silica gel). Since the number of Si-O-Si bond located  Sand grains (QS) with a low SSA also have Si-OH groups at the particles surface, which of course affect the binding strength of the neighbored Si-O-Si bond. But the low porosity and the relatively large particle size of such grains result a low SSA such that the number of Si-O-Si groups located near to the surface relative to the ones located within the particles center (Fig. 2) is much smaller for quartz sand as compared to amorphous silica like Sipernat. Consequently, mostly the Si-O-Si groups located within the particles center will show up in the FTIR spectra resulting in a small Si-O-Si band. Additionally, quartz has the highest internal order compared to the other materials analyzed which results in a narrow Si-O-Si band.
In the FTIR of quartz sand (black line in Fig. 1) the second maximum at the left-hand side of the Si-O-Si band (grey arrow, Fig. 1) can be explained by the fact that the quartz sand studied here is a commercially available material that is obtained by ashing and HCl treatment of sea sand. However, this procedure will not remove cations that are fixed within the Si-O-Si structure. Thus, the quartz sand will contain small amount of cations like iron within its Si-O-Si structure in contrast to silica (Fig. 5d) Like ZEOfree the Imogolith (Fig. 3b, (Fig. 3b) indicative for Si-O-Al bands 32 , Wada et al. 33 but, these bands did not did not show a second maximum for the Si-O-Si band (Figs. 3b and 3c). The same study found that increasing Al content increases a shift in the Si-O-Si band by comparing the FTIR spectra of SROAS with different Si:Al ratios (Fig. 6a). A similar shift was observed by Lenhardt et al. 20 when analyzing short-ranged ordered aluminosilicates with varying Al:Si ratios (Fig. 6b). Stein et al. 19 observed a peak shift towards lower WN and a broadening of the entire envelope (1000-1300 cm −1 ) when analyzing silica metal compounds which were synthesized at undersaturation of silicates Figure 6. WN 800-1200 cm −1 sections in FTIR spectra of (a) soil allophanes and imogolith (adopted from Wada et al. 1979), (b) short-range ordered aluminosilicates SROAS with different Al:Si ratios (adopted from Lenhardt et al. 20 ), and (c) polymerized silica associated with Cu 2+ (adopted from Stein et al. 19 ). www.nature.com/scientificreports/ (Fig. 6). As the definition of allophane by Parfitt 32 is wrong, we suggest that those compounds may be simple SROAS, amorphous aluminosilicas or ASi. The FTIR spectra of the water soluble Sipernat 50 s fraction (Fig. 4d) showed an additional band that is in the WN range of the Si-O − band found for ZEOfree (1500 cm −1 ; Fig. 4a) and Na 4 SiO 4 (1475 cm −1 ; Fig. 4d) compared to that of the bulk Sipernat (Fig. 4b). This band indicated that the FTIR spectral signature of water soluble Sipernat 50 s-fractions can be distinguished from that of the bulk Sipernat 50 s, and suggests the water soluble fraction of Sipernat 50 s consist mostly of small sized amorphous, anionic silica particles, The pH values is about 6, such deprotonation of silanol groups may be neglectable small. The bands at 1400 and 1100 cm −1 in the FTIR of the water soluble SIP50s fraction possibly indicate the presence of dimeric silica, since they correspond to the respective ones in the FTIR of a dimeric organo-silica (adopted from Igarashi et al. 36 ; Fig. 4e).

Conclusion
As Si cycling, its speciation, and availability in soils is crucial to terrestrial ecosystem functioning and services the analysis of different Si species is highly important. Comparing amorphous silica, SROAS and silicic acid species using FTIR, we aimed to identify absorption bands that are characteristic for these different Si phases analyzed. Based on our results we report here that SROAS and Si phases containing traces of other elements in their Si-O-Si network can be distinguished from pure amorphous Si phases as their band positions and peak shapes of the respective IR spectra are specific for the bonds within the materials analyzed. Furthermore, liquid phases of silicic acid/water-soluble Si phases can be distinguished from the respective bulk material as water-soluble fractions showed an additional peak in its IR-spectra. As the current methods to analyze different Si phases are based on extraction procedures, which are not very selective 15 , we recommend using FTIR analyzes for such analysis. The limited number of synthetic samples and/or soil restricts generalization of our results regarding the estimation of differing Si phases in soil. Thus, further investigations using soils of varying properties with regard to their Si biogeochemistry are important to verify the described method and thus, overcome soil analyzes which are using unselective extraction agents to answer specific questions in terms of Si and its role for ecosystem functioning.

Data availability
The datasets used and/or analyzed during the current study is available from the corresponding author on reasonable request.